A usual laundry dryer comprises a substantially parallelepiped-shaped outer box casing and a cylindrical laundry drum for housing the laundry to be dried. The laundry drum is arranged in axially rotating manner inside the casing and rotates about its horizontally oriented longitudinal axis. A front opening for loading and unloading laundry is in the front face of the casing. A door is hinged to the front face of the casing in order to close the front opening.
The laundry dryer of the above type comprises a closed-circuit hot-air generator designed to circulate inside the revolving laundry drum a stream of hot drying air with low moisture content. The drying air stream flows through the revolving laundry drum and over the laundry inside said drum in order to rapidly dry the laundry. The hot-air generator comprises an air recirculating conduit having its two ends connected to opposite sides of the revolving laundry drum. A radial air stream fan is arranged in the recirculating conduit and generates the drying air stream through the revolving drum. A heating device is provided for warming up the drying air stream before said drying air is supplied to the laundry drum. A moisture condensing unit is provided for removing moisture from the drying air stream after said drying air stream has left the laundry drum.
One embodiment of the moisture condensing unit may be formed as an air-to-air type heat exchanger, through which the drying air stream flows. A cooling air stream is conveyed into the laundry dryer, passes through the air-to-air type heat exchanger and is exhausted from the laundry dryer by means of an open cooling air circuit. A cooling air fan is installed within the cooling air circuit. The heating device for warming the drying air before said drying air is supplied to the laundry drum consists in one or more electric heaters.
Another embodiment of the hot-air generator comprises a heat-pump device including a compressing device and two heat exchangers. The compressing device subjects a gaseous refrigerant to compression, e.g. an adiabatic compression, so that the pressure and temperature of the refrigerant are much higher at the outlet than at the inlet of the compressing device. The heat exchangers are arranged one after the other along the drying air stream recirculating conduit and are designed to receive the refrigerant from the compressing device in order to cool down or heat up the airflow, respectively. In detail, a first air-to-refrigerant heat exchanger, commonly referred as the evaporator, is provided for rapidly cooling down the airflow arriving from the revolving laundry drum in order to condense the surplus moisture in the airflow. Further, a second air-to-refrigerant heat exchanger, commonly referred to as the condenser, is provided for rapidly heating up the airflow arriving from the first heat exchanger and directed back to the revolving laundry drum, so that the airflow re-entering into the revolving laundry drum is heated up rapidly to a temperature higher than or equal to that of the air stream flowing out of the revolving laundry drum.
In the above types of domestic laundry dryers, the flow of the drying air stream through the revolving laundry drum is accomplished by an electric motor that drives both the radial air stream fan and a transmission system for rotating the revolving laundry drum. Such motor may be driven by an inverter circuit for controlling motor operating conditions. Usually, software controlling motor operating conditions works independently from actual drying air stream, which can be strongly influenced by the cleanness status of air filters used to catch fluff transported by the drying air stream. The fluff is dust produced by laundry during the drying process. With the continuous accumulation of fluff on the air filters the flow of the drying air stream decreases, while the speed of the air stream fan remains the same for the whole drying process. Thus, the efficiency of the heat pump is affected. Compared to the situation with perfectly clean air filters, the condensing temperature rises more quickly and the maximum heat pump refrigerant fluid temperature threshold is reached rather earlier. Therefore a cooling fan of the compressor device is also activated earlier in order to avoid overheating of the heat pump system. These drawbacks negatively affect the drying performances, because the time cycle is longer and the final energy consumption is higher. At last, average energy consumption measured according to the Standard Energy Label is higher than a drying process carried out with perfectly clean air filters.
EP 2 487 290 A1 discloses a home laundry dryer. The output motor speed is maintained substantially equal to a predetermined speed value, which is associated to a nominal airflow rate within the air recirculating conduit and to a nominal drum speed. Notwithstanding such provisions the energy consumption of a drying process is still undesirably high. In addition, the operating conditions of the laundry dryer are not linked to the actual status of laundry during a drying process.
EP 2 468 950 A1 discloses a process and machine for drying laundry. The motor speed of the laundry drum is controlled in dependence of laundry dryness. The speed of the laundry drum is higher than the lowest speed in order to hold the laundry adhering to a wall of the drum. Therefore such provision is rather useful during a laundry cooling phase than during a drying phase with hot drying air in which it is required that laundry may move through the drum inner chamber so as to intercept the drying air flow.